Bloomberg notes today that regulator filings reveal Berkshire's muni-bond holdings almost doubled in just nine months, rising to $4.05 billion as of March 31, compared to $2.05 billion on June 30, 2008.

The article recalls Buffett's own amazement at sky-high muni yields as the credit crunch was at its worst late last year and early this year. In his February letter to shareholders, Buffett wrote:

"The investment world has gone from underpricing risk to overpricing it. A few years ago, it would have seemed unthinkable that yields like today's could have been obtained on good-grade municipal or corporate bonds even while risk-free governments offered near-zero returns on short-term bonds."

Dear Mr. Buffett author Janet Tavakoli points out that smart investors don't limit their search to Wall Street when they're looking for bargains. Referring to the muni bond market, she says, "There have been some excellent bargains for people like Warren Buffett, who understands that the principle of value investing don't change, no matter the circumstances."